The present invention relates to a burst signal generating circuit in a video processing system which receives a discontinuous burst signal and generates a continuous burst signal by using a phase locked loop.
Generally, a video processing system processes a video signal by using a color burst signal. An example of a color burst gate circuit used to obtain a color burst signal is shown in U.S. Pat. No. 4,549,225 issued Oct. 22, 1985 to Watanabe and entitled COLOR VIDEO SIGNAL PROCESSING CIRCUIT FOR PERFORMING LEVEL CONTROL AND TIME AXIS DEVIATION COMPENSATION.
On the other hand, when a digital video signal is processed in the video processing system, the discontinuous burst signal is detected from a composite video signal and then the continuous burst signal is generated to process the video signal.
FIG. 1 is a conventional circuit diagram for generating a burst signal. This circuit diagram is composed of a burst detector 10 for inputting the composite video signal and for detecting and outputting the discontinuous burst signal by a burst gate pulse; a phase detector 12 which receives the discontinuous burst signal detected from the burst detector 10 and a given continuous burst signal, and compares a phase difference and generates an error signal corresponding to the phase difference; a loop filter 14 for integrating the error signal outputted from the phase detector 12 and for outputting a DC (direct current) value; a sample and hold part 16 for inputting the DC value integrated from the loop filter 14 and for outputting the integrated error value in a burst area of the burst gate pulse, and for holding the error value except in the burst area of the burst gate pulse; and a voltage controlled oscillator 18 for inputting a signal outputted from the sample and hold part 16 and for generating the burst signal continued by an oscillating frequency.
Referring to the operation of the circuit of FIG. 1, when the composite video signal such as 20 of FIG. 2 is inputted through an inputting terminal P1, the burst detector 10 detects the discontinuous burst signal such as 22 of FIG. 2, and then outputs the discontinuous burst signal to the phase detector 12. The phase detector 12 compares the phase difference between the discontinuous burst signal detected from the burst detector 10 and the given continuous burst signal. Then, the phase detector 12 generates the error signal corresponding to the phase difference and outputs the error signal to the loop filter 14. The loop filter 14 integrates the error signal outputted from the phase detector 12 using a filter and outputs the integrated DC value to the sample and hold part 16. The sample and hold part 16 inputs the DC value integrated from the loop filter 14. Then, the sample and hold part 16 outputs the integrated error value in the burst area of the burst gate pulse such as 24 of FIG. 2, and holds the error value in the area except the burst area of the burst gate pulse, then outputs the error value to the voltage controlled oscillator 18. The voltage controlled oscillator 18 inputs the signal outputted from the sample and hold part 16, and generates and outputs the continuous burst signal such as 26 of FIG. 2 by the oscillating frequency.
In the aforementioned conventional circuit for generating the burst signal, there is difficulty in selecting the loop gain and loop bandwidth, since the performance characteristics of the phase locked loop are directly influenced by the sample and hold part 16.